To improve the characteristics of steerability at high speed (comfort, ease with which it lifts out of the water, how well it holds its heading, . . . ) whilst at low or medium speeds maintaining correct immersion of the propeller, it has already been proposed (FR 2,398,660, FR 2,510,064) that, at least in their substantially parallel part, each of the two branches of the tube should, in cross section, exhibit a multi-lobed, especially two-lobed shape, the major axis of which is inclined, and in which the upper compartment exhibits a larger cross section than the lower compartment; furthermore, a lateral sheet is fixed tangentially to at least some of the lobes of the outer side of the branch in question, and in particular is fixed tangentially to the large upper lobe and to the small lower lobe of the two-lobed structure, these lateral sheets constituting lateral lift planes on which the upthrust is exerted.
Small craft of this type are completely satisfactory and have been very widely developed.
However, the multi-lobed structure of each branch is complicated to manufacture and is therefore expensive. For this reason, this improved structure has hitherto been reserved for top-of-the-range craft.
However, there is now a demand for small craft at the bottom or in the middle of the range which exhibit the aforementioned steerability advantages whilst retaining a manufacturing cost which is lower than that which the use of a multi-lobed structure as described above entails.